The present invention relates to a ribbon mask for a dot matrix impact printer In the dot matrix impact printer, the ribbon mask is provided on a carriage between an inked ribbon and a printing paper for preventing the ribbon from coming into contact with the paper.
Japanese Utility Model Applications Laid-open Nos. 61-170561 and 55-45540 disclose a ribbon mask for a printer in which a lower end of the ribbon mask is secured to a member mounted on a carriage (first prior art). Japanese Utility Model Application Laid-open No. 61-138558 discloses a ribbon mask in which both lateral ends of the ribbon mask are engaged with grooves formed in ribbon guide bars vertically provided on a carriage and a lower end of the ribbon mask is engaged with an engaging portion provided on the carriage (second prior art). Japanese Utility Model Application Laid-open No. 59-46262 discloses a ribbon mask in which both ends of the ribbon mask is secured to a ribbon guide (third prior art).
If the ribbon mask is moved to the paper during the printing, the paper will be stained with ink. In order to prevent the contact of the inked ribbon with the paper, the ribbon mask must be accurately positioned with respect to the print head.
For example, if the distance (A) between tip ends of print wires of the print head and a platen is 0.40 mm, the thickness (B) of the printing paper is 0.10 mm, the thickness (C) of the inked ribbon is 0.12 mm, and the thickness (D) of the ribbon mask is 0.08 mm, accuracy for the position of the ribbon mask is EQU A-(B+C+D)=0.10 mm.
This is beyond the capability of a manufacturing machine.
The position of the ribbon mask of each of the prior art references described above is influenced by the following factors. In the first prior art, the accuracy of the position of the ribbon mask depends on positional accuracy at the mounting of the ribbon mask, and flatness and upright angular accuracy of the ribbon mask. In the second prior art, it depends on positional accuracy at the mounting of the ribbon guide bars, accuracy of the grooves in the guide bars and flatness of the ribbon mask. In the third prior art, it is influenced by the positional accuracy of the ribbon guide, and positional accuracy of the ribbon mask.
As described above, in the above prior art; since the position of the ribbon mask is dependent on accuracies of members for mounting the ribbon mask, the members must be manufactured and assembled to a strict precision.